This disclosure relates to a switched-mode power supply device.
A switched-mode power supply device described, for example, in Japanese Patent Application Publication No. 2008-92794 (Patent Literature 1) is known as a switched-mode power supply device of prior art. The switched-mode power supply device described in Patent Literature 1 has first to third duty cycle control modes, and a control circuit enters the first duty cycle control mode (heavy load mode) when the value of a feedback signal is IC1. In the first duty cycle control mode, the switching frequency is adjusted to a 100% value on average. When the value of the feedback signal is IC2 (greater than IC1), the control circuit shifts from the first duty cycle control mode to the second duty cycle control mode (light load mode; green mode).
In the second duty cycle control mode, the control circuit adjusts a peak switch current to a fixed value and adjusts the switching frequency to below 100% when the feedback signal increases. The switching frequency reaches a threshold value, which is 20% of the 100% value when the value of the feedback signal reaches IC3 (greater than IC2). When the value of the feedback value is IC3 (greater than IC2), the control circuit shifts from the second duty cycle control mode to the third duty cycle control mode.
With the first and second duty cycle control modes, the drain of the switching element can be suppressed.
However, in the entire first duty cycle control mode and part of the second duty cycle control mode, the switching frequency is high and therefore the switching loss of the switching element is large. For this reason, it has been difficult to further make the switching frequency higher.
Also, as illustrated in FIG. 9, in the prior-art switched-mode power supply device, feedback voltage FB linearly decreases with decrease in load power P0 (load ratio). For this reason, the switching frequency line linearly decreases with decrease in load power P0 (load ratio) also during a light load state. With this straight switching frequency line, the frequency is high and the switching loss therefore increases during a moderate load state. Accordingly, the efficiency is lowered.
Also, there has been a demand to further make the switching frequency higher to downsize a transformer. However, if the switching frequency is further made higher, the switching loss lowers the efficiency during a heavy to moderate load state.